MIXING COMPONENTS AND HYBRIDIZATION PROCESSES OF KALAMA PERIOD INTERMEDIATE MAGMAS FROM MOUNT ST. HELENS: EVIDENCE FROM MAFIC PHENOCRYSTS

A series of eruptions producing an extraordinary dacite-andesite-dacite stratigraphy occurred over less than 165 years during the Kalama period (1479-1750 A.D.) at Mount St. Helens. This rapid and strong change in composition has previously been attributed to the mixing of basaltic magma with dacitic magma to make andesite and mineral assemblages are consistent with this. Mafic phenocrysts of dacitic origin are amphibole and orthopyroxene, while those phenocrysts originating from basaltic magma are olivines and some clinopyroxenes. We have investigated mafic silicate phenocrysts from the andesitic middle Kalama period and likely dacitic and basaltic endmembers (early and late Kalama period lavas and tephras as dacitic endmembers; lavas of Castle Creek period, 1895-2550 years b.p., as basaltic endmembers) to detail magma components and mineralogical consequences of hybridization. The initial response to mixing is some resorption of olivine, the crystallization of clinopyroxene, and an overgrowth on pre-existing crystals in form of more Mg-rich rims on orthopyroxene and less Mg-rich rims on Cr-rich, high Mg# clinopyroxene. Prolonged exposure to a hybrid melt causes complete resorption of olivine and the breakdown of amphiboles into pseudomorphs that subsequently are overgrown by clinopyroxene. Compositions of amphiboles indicate an additional source beside those originating from magmas similar to dacites of early and late Kalama age. A dacitic X-tephra banded pumice layer has a bulk composition similar to early Kalama period dacite (e.g. Wn pumice), but contains resorbed olivine grains and orthopyroxene crystals with Mg-richer rims, although less pronounced than in Kalama andesites. Altogether, this is consistent with mixing, yet the influx of basalt was only 5% compared to ~60% in the andesitic hybrids. Middle Kalama period tephras and lavas do not likely represent a single magma batch in which phenocrysts crystallized or reacted progressively through time but rather represent individual magmas with different compositions, different mixing members and proportions, and erupting at different hybridization stages. Mafic input similar in LREEs and MREEs to some Castle Creek period basalts is likely to have been a mixing component during the middle Kalama period.